In this study, PBI-based block copolymers were developed and their performance as membranes in high temperature polymer electrolyte membrane fuel cells was evaluated. This type of block copolymer consists of phosphophilic PBI and phosphophobic non-PBI segments. The final properties of such block copolymers strongly depended on the length of the individual blocks and their chemical structures. In a systematic approach, a series of various block copolymers was synthesized and characterized both in terms of ex situ properties (e.g., proton conductivity, phosphoric acid uptake, swelling behavior) and in situ fuel cell tests. A very poor membrane-electrode interface limited the performance of the membrane electrode assemblies, but was remarkably improved in power output, stability, and long-term durability by treating the electrode interface with a fluorinated PBI derivative. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1831-1843